1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to satellite digital audio service (SDARS) receiver architectures.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Satellite radio operators will soon provide digital quality radio broadcast services covering the entire continental United States. These services intend to offer approximately 100 channels, of which nearly 50 channels will provide music with the remaining stations offering news, sports, talk and data channels. According to C. E. Unterberg, Towbin, satellite radio has the capability to revolutionize the radio industry, in the same manner that cable and satellite television revolutionized the television industry.
Satellite radio has the ability to improve terrestrial radio""s potential by offering a better audio quality, greater coverage and fewer commercials. Accordingly, in October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The FCC allocated 25 megahertz (MHz) of the electromagnetic spectrum for satellite digital broadcasting, 12.5 MHz of which are owned by CD Radio and 12.5 MHz of which are owned by the assignee of the present application xe2x80x9cXM Satellite Radio Inc.xe2x80x9d. The FCC further mandated the development of interoperable receivers for satellite radio reception, i.e. receivers capable of processing signals from either CD Radio or XM Radio broadcasts.
The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters will broadcast the same program content in order to improve coverage reliability. Some mobile receivers will be capable of simultaneously receiving signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation against multipath and blockage of the satellite signals. In accordance with XM Radio""s unique scheme, the 12.5 MHz band will be split into 6 slots. Four slots will be used for satellite transmission. The remaining two slots will be used for terrestrial re-enforcement.
Nonetheless, a need remains in the art for a frequency plan effective to ensure continuous reception in the presence of blockages and other sources of interference. There is a further need for a receiver adapted to receive signals transmitted in accordance with such a frequency plan.
The need in the art is addressed by the system and method of the present invention. In accordance with the present teachings, the assigned 12.5 MHz bandwidth (hereinafter the xe2x80x9cXMxe2x80x9d band) is partitioned into two equal ensembles or program groups A and B. The use of two ensembles allows 4096 Mbits/s of total user data to be distributed across the available bandwidth. Each ensemble will be transmitted by each satellite on a separate radio frequency (RF) carrier. Each RF carrier supports up to 50 channels of music or data in Time Division Multiplex (TDM) format. With terrestrial repeaters transmitting an A and a B signal, six total slots are provided, each slot being centered at a different RF carrier frequency.
The use of two ensembles also allows for the implementation of a novel frequency plan which affords improved isolation between the satellite signals and the terrestrial signal when the receiver is located near the terrestrial repeater. In accordance with the system and method of the present invention, first and second carrier frequencies are transmitted from a first transmitter. The first carrier frequency is at a first low end of a band and the second carrier is at a second higher end of the band. Third and fourth carrier frequencies are transmitted from a second transmitter. The third carrier is at the lower end of the band but higher in frequency than the first carrier and the fourth carrier is at the higher end of the band but lower in frequency than the second carrier. Fifth and sixth carrier signals are transmitted from a third transmitter. The fifth carrier signal is higher in frequency than the third carrier signal and the sixth carrier signal is lower in frequency than the fourth carrier signal, but higher in frequency than the fifth carrier signal.
In the illustrative embodiment, the first transmitter is located on a first satellite, the second transmitter is located on a second satellite and the third transmitter is located on a terrestrial repeater. The information modulated onto the first, third and fifth carriers is identical such that the first, third and fifth carriers comprise a first ensemble. The information modulated onto the second, fourth and sixth carriers comprise a second ensemble.
In the illustrative embodiment, the inventive system includes plural receivers adapted to simultaneously receive first and second ensembles. The first ensemble includes the first carrier transmitted from a first satellite, the third carrier transmitted from a second satellite and the fifth carrier transmitted from a terrestrial repeater. Likewise, the second ensemble includes the second carrier transmitted from the first satellite, the fourth carrier transmitted from the second satellite and the sixth carrier transmitted from the terrestrial repeater. The inventive receiver further includes a mechanism for selectively outputting signals transmitted within the first and second ensembles.
As a result, the carriers assigned to the second satellite isolate the carriers assigned to the first satellite from the carriers assigned to the terrestrial repeater, allowing the reception of carriers assigned to the first satellite in the event the carriers assigned to the second satellite are interfered with by the carriers assigned to the terrestrial repeater. Thus, the terrestrial repeater may then transmit a signal inband which contains information received from the first satellite without concern that the terrestrial signal might interfere with the reception of the satellite signals by the receivers colocated with the terrestrial repeater. This allows for the use of practical and available components to implement the terrestrial repeater which repeats a broadcast signal received from an inband satellite signal.